1. Field of the Invention
The present invention relates to an optical head for a laser printer, and more particularly to an optical head for a laser printer which includes no movable parts therein and in which a light source, an optical guide, an optical modulator, and the like are integrated to reduce the size.
2. Description of the Prior Art
A demand for a high-performance printer has increased with the enhanced functions of office apparatuses and the like. Particularly, a laser printer using a semiconductor laser as a light source, which has advantages such as reduced noise at printing and image forming with high resolution, is expected to be further improved for more sophisticated performance.
FIG. 5 is a schematic diagram of an optical head and a portion adjacent thereto of a conventional typical laser printer. In this laser printer, a modulated light beam 42 emitted from a semiconductor laser 41 is converged by a collimator lens 45 to be received by a polygon mirror 43, where the light beam 42 is deflected by rotation of the polygon mirror 43. The deflected light beam 42 is then converged again by a focus lens f.sub.0, a cylindrical lens 47, etc. for scanning on a photosensitive drum 44. Thus, the semiconductor laser 41, the collimator lens 45, the polygon mirror 43, the focus lens f.sub.0, the cylindrical lens 47, etc. constitute an optical head which emits light beams to the photosensitive drum 44. Only parts on the photosensitive drum 44 which are illuminated by the light beam 42 from the optical head are charged with electricity, to which toner attaches (not shown). The toner attaching on the photosensitive drum 44 is transferred to a sheet 48 which is conveyed synchronously with the rotation of the photosensitive drum 44. The toner transferred to the sheet 48 is then heat-treated so as to be fixed on the sheet 48, resulting in printing an intended image on the sheet 48.
An optical head for a laser printer using a hologram plate instead of the polygon mirror 43 has also been developed. Both optical heads can effect high resolution and high speed printing.
A different type of optical head from the above ones employing mechanical scanning has also been developed, in which a number of minute light emitting diodes (LED) are linearly arranged at intervals of as narrow as about 50 .mu.m to form an LED array. In this type of the optical head, each LED is individually modulated and emits light beams to illuminate a photosensitive drum.
Further, a liquid crystal optical head for a laser printer has been developed, in which an optical modulator using a liquid crystal and disposed in front of a photosensitive drum modulates light beams radiated from a light source and transmits them to the photosensitive drum. In this optical modulator, light transmittance of each minute square of a grating of the liquid crystal is controlled so as to individually modulate light beams at each square. Furthermore, in order to solve the problem of the low responding speed of the liquid crystal in the above optical modulator, optical modulation by a magneto-optical shutter has been studied for practical application.
However, in the first type optical head which employs mechanical scanning, a high speed rotation part is required, causing the troubles of noise and wear. Also, since a certain amount of space is inevitably necessary for the scanning by light beams, it is difficult to reduce the size.
In the second type optical head which uses the LED array, more than 2,000 LED's are required, and each of such a large number of LED's must be controlled so as to attain uniform luminance thereof and thereby to minimize the possibility of nonuniformity of a printed image. Also, since such a large number of LED's must be set in array, it is difficult to attain high yield and prolonged life, thus having difficulty for practical use.
In the third type optical head which uses the liquid crystal shutter or the magneto-optical shutter for optical modulation, a light source such as a fluorescent lamp, a group of lenses to control light beams from the light source, an optical modulator, etc. must be arranged separately from each other when fabricated. This requires complicated adjustment among these components at fabrication, as well as making it difficult to reduce the size.